This invention relates to an electronic circuit for use in a time division multiple access (often abbreviated to TDMA) system and, in particular, to an electronic circuit comprising a modulator.
As a time division multiple access system, proposal has been made of a multidirectional time-division multiplex radio communication system using a comparatively small quantity of communication channels. Such a communication system comprises a central station fixedly located at a predetermined terrestrial site and a plurality of substations geographically spaced on the earth from the central station, as will later be described with reference to one of about twenty fingers of the accompanying drawing.
The central station simultaneously transmits a TDMA signal to all of the substations with each channel allotted in the TDMA signal to each of the substations. Responsive to the TDMA signal, each substation derives information only from the channel allotted thereto.
On the other hand, each substation transmits information to the central station in the form of a burst through a channel allotted to the substation. The burst is subjected to modulation, such as quadrature amplitude modulation.
A linear modulator, such as a balanced modulator, is used in combination with a baseband filter in each substation to carry out modulation. This enables a radio frequency filter of a wide pass band to be connected to the modulator in each substation because undesired components are reduced as compared with a modulator of a switched type.
As will later be described with reference to another figure, the burst should be produced by switching a data signal sequence in accordance with a burst control pulse having steep leading and trailing edges. As a result, harmonic components resulting from the leading and trailing edges are included in the burst without being removed by the radio frequency filter. Such harmonic components fall in any other receiving frequency bands in the form of interference waves. Hence, the receiving frequency bands are subjected to interference due to the interference waves. The influence of interference undesiredly grows as desired signals are reduced in magnitude by fading.